This invention relates to a pneumatic booster adapted for use in a hydraulic braking system of a vehicle such as an automobile wherein a depressing force applied on a brake pedal is multiplied by utilizing pneumatic pressure.
One prior art pneumatic booster comprises a housing, a power piston partitioning the interior of the housing into a constant pressure front chamber and a variable pressure rear chamber, a return spring disposed in the front chamber and biasing the power piston rearwards, a reaction disc mounted on the power piston, an output shaft disposed frontwards of the reaction disc, an input shaft disposed rearwards of the reaction disc, and a valve mechanism associated with the input shaft and controlling the pressure in the rear chamber. The front chamber is usually connected to a source of vacuum pressure such as an intake manifold of an engine, and atmospheric pressure is introduced into the rear chamber when the valve mechanism is actuated thereby generating a differential pressure between rear and front chambers. The reaction disc acts to transmit the output force from the power piston to the output shaft and, to transmit reaction force of the output force to the input shaft.
A conventional pneumatic booster has so-called "jump-in" characteristics as shown in FIG. 3 such that the output force does not act on the output shaft until the input force increases to a predetermined amount. Such "jump-in" characteristic assures smooth and reliable characteristics of the output force irrespective to such as mechanical loss in the pneumatic booster. Recently, it is desired to increase the amount of "jump-in" so as to improve the feeling on a brake pedal and to increase the initial brake applying force to match the increase in the running speed of the vehicle.
Heretofore, the amount of "jump-in" has been adjusted by changing the clearance between the reaction disc and the forward end of the input shaft, namely, in increasing "jump-in" the clearance was increased. However, when the clearance is increased, the deformation of the reaction disc increases which is hazardous to the durability of the reaction disc.